1. The Field of the Invention
The present invention relates to improved printed circuit boards, and more particularly, to non-metallic sealant elements covering contact surfaces on printed circuit boards.
2. The Relevant Technology
Since printed circuit boards replaced discrete wiring system, there has been a deluge of technology relating to printed circuit boards, and specifically, to the electrical features contained within circuit boards. Printed circuit boards integrated within PC cards provide the necessary interconnect for circuitry to perform its intended electrical functions. For example, in one type of PC card, the circuit board comprises electronics forming a modem that enables the host to receive and transmit information over telephone lines. In another PC card, the circuit board with its electronic components enables the host to receive and transmit information with a network system.
Printed circuit boards are comprised of conductive interconnections or traces that provide conductive xe2x80x9cwiringxe2x80x9d between components. These conductive interconnects or traces, usually made of copper, are etched from metallic planes on the printed circuit board using known techniques such as photographic and chemical processes. Because of the desire for low-resistance interconnections, these metallic interconnects are generally comprised of copper or related alloys which provide low-resistance at a reasonable cost.
Those familiar with conductive metals, such as copper, appreciate that some conductive metals, and in particular copper, tend to react forming oxidation in an ambient air environment. In fact, copper used on the contact surfaces of printed circuit boards tends to oxidize rapidly. If left unsealed from the ambient air, copper oxidizes forming a less conductive electrical interface for subsequent connector or contact mating, and the oxidation also results in easily detachable copper oxides that are undesirable debris throughout the printed circuit board and surrounding electronics.
While it is known that exposed conductive surfaces may be sealed using many insulative compounds, many surfaces must remain accessible and electrically conductive so that electrical communication between contact surfaces may be achieved and maintained. Thus, conductive sealants are essential for covering those conductive areas that require durable and reliable electrical interfaces.
In addition to the need that contact surface sealants be electrically conductive and thereby exhibit low resistance, sealants covering conductive surfaces that physically engage with other contacts such as connector terminals and the like must be further capable of withstanding repeated physical engagements. For example, physical conductive engagements occur when printed circuit boards having a card-edge connectors are inserted and removed from receiving connectors or jacks. This physical friction-based connector mating requires that any conductive sealant for conductive areas assume a sufficiently xe2x80x9chardxe2x80x9d surface that is not easily marred and removed. If such degeneration of the sealed contact surface occurs, then oxidation resumes and conductive debris flakes-off and contaminates and may induce electrical shorts on the printed circuit board and surrounding electronics.
One prominent solution for sealing conductive surfaces involved in continuing physical interfacing has been to apply, usually in the form of plating, other non-oxidizable or less-oxidizable metals to the underlying conductive metal. For example, gold and other heavy metallic elements have been used to coat the copper contact surfaces while maintaining conductivity of the underlying copper contacts. Typically, the copper is plated with a layer of gold, which has an underlayer of nickel. After the gold is applied by plating, it is masked with tape or other physical barrier to prevent any contaminating solder reflow during the application of the electronic components to the printed wiring or electrically conductive traces on the printed circuit board.
Those familiar with plating processes appreciate the various undesirable side-effects of plating and particularly gold plating on small dimension electronic printed circuit boards. For instance, plating copper with gold is expensive because it is process-intensive and involves procedures that may be volatile and result in reduced quality. Gold is also not optimally environmentally sound because a number of toxic chemicals are used to process the gold and are left behind as dangerous by-products.
Furthermore, it is also difficult to plate contacts with metals such as gold and maintain alignment of the sliding contact on top of the etched contact surfaces of the printed circuit board as the plating process results in an additional layer of metal that exhibits sharp profile edges. The nature of copper etching leaves the copper with sharp vertical edges, and when gold is applied, this edge becomes even more pronounced. These edges can affect finely dimensioned interfaces that employ a sliding contact with the plated surface.
For example, a sliding contact interface generally results from a physical sliding of a contacting tab across the plated contact surface. When fine dimensions are involved, the contacting tab and the plated contact surface cannot be subjected to appreciable variations in tolerances otherwise the contacting tab xe2x80x9cslipsxe2x80x9d off the sharp edge of the plated contact surface during the insertion of the connecting tab with the plated contact surface and cannot return to a centered orientation on the plated contact surface due to the steep conductive edge formed on the contact surface by the plating process. That is to say, if the sliding contact becomes misaligned, it is unable to surmount the sharp edges and realign itself. Thus, a need exists to adequately cover the contact surfaces with an element having a more gradually sloping edge. Additionally, if misalignment does occur, the sliding contact should more easily overcome the sloped edges and realign itself.
Others have attempted to solve the problem associated with copper sliding contact surfaces by varying etch widths of the copper, varying the printed circuit board thickness, altering the assembly process, re-configuring contact surfaces, and tightening tolerance controls. Unfortunately, these attempts have been to no avail.
A non-metallic conductive structure, which covers the slidable contact surfaces on printed circuit boards and; (i) enables sufficient hardness, (ii) maintains adequate conductivity, (iii) maintains better alignment between sliding contact surfaces, (iv) reduces oxidation of the contact surfaces and brings about these benefits at a lower cost and with more environmentally-sound techniques than is provided by the available structures and processes.
Thus, in a preferred embodiment, the non-metallic conductive sealant coats the conductive printed circuit board trace, such as a copper contact surface of the printed circuit board. This non-metallic conductive sealant is comprised of a carbon-based ink composition that is preferably applied using a printing process such as a silk-screen process.
The second sliding contact surfaces comprise conductive pins or tabs that interface with the processed traces or contact surfaces on the printed wiring board. One such implementation of a sliding contact is present on a communication card employing a retractable/extendable communication jack such as an XJACK(copyright) or other electrical interface. When the extendable jack contact surfaces slide on top of and against the contact surfaces of the printed circuit board, an electrical communication is created. The carbon ink does not insulate the contact surfaces of the printed circuit board, but rather enables conductivity to pass from the underlying conductive copper contact surface to the sliding contact surface.
The use of carbon ink is also more cost-effective and environmentally sound than the use of gold. The application and cost of carbon ink is approximately one-fifth of the price of gold electroplating. The environmental benefits are also substantial. Carbon ink does not have the hazardous side products that are inherent with gold electroplating. Further, carbon ink is more easily applied to contact surfaces, which reduces or eliminates altogether the possibility of solder splash.
Additionally, carbon ink has desirable characteristics that make it well suited for its application onto copper contact surfaces. For instance, carbon ink has a greater hardness than gold. Also, carbon ink assumes more gradually sloping edges upon application, which make misalignment less probable and problematic.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.